What is a Standard Illuminant (Reference Illuminant)?

Reference illuminants are a set of standardized spectral power distribution (SPD) data sets that represent real light sources such as incandescent light, sunlight, and fluorescent light. The reference illuminants (also called standard illuminants) represent the mathematical tables of values (optical power versus wavelength) used for light measurement (colorimetry) and characterization. Typically, all illuminants are compared against the spectral energy distribution of the corresponding blackbody radiator, an artificial temperature radiator that emits light with a specific color at defined temperatures (correlated color temperature, or CCT), measured in Kelvin (K).

For scientists and lighting engineers it became necessary to establish a set of standard, defined light sources with tabulated SPDs that could be used as a reference point to compare lighting sources of all kinds. The first (Illuminant A) was defined in 1931 by the International Commission on Illumination (CIE), with additional ones added subsequently.

The CIE Illuminants

CIE illuminant standards average the spectral power distribution (SPD) of as many specimens of a brand of lamp as the lab has available. CIE established the first standard illuminant, Standard Illuminant A, in 1931. The organization has added other standards as technology progressed through the years, including in 2018, with the introduction of LED illuminants. [1] D65 is now the main reference illuminant. The major reference illuminants are summarized in the table below.

	Illuminant	Description
	A	The first standard established by CIE, standard A, as the document claims, “represents typical, domestic, tungsten-filament lighting” with correlated color temperature (CCT) of 2856 K.
	B	The second standard introduced in 1931, standard B represents direct sunlight by adapting standard A with a liquid filter. The result is a noon-time equivalent with a CCT of 4874 K. The representation was considered poor, with limited ultraviolet and short-visible wavelengths. It was deprecated in favor of D series illuminants.
	C	Also introduced in 1931, Illuminant C is a modified version of Illuminant A and is intended to represent average noon-time sunlight with a CCT of 6774 K. Like standard B, the representation is considered poor, with limited ultraviolet and short-visible wavelengths. It was also deprecated in favor of D series illuminants. Despite being deprecated, it merited mention in CIE, Publication 15:2004 because it represents values still useful in calculations and calibrations. The filter developed for illuminant C includes properties that filter infrared from incandescent lights.
	D65	Now the principal reference illuminant, D65 displays average daylight (including ultraviolet wavelength region) with a CCT of 6504 K. It is one of the D series, which represent different types of daylight, as mathematical models.
	E	Illuminant E is an “equal energy” model and a theoretical reference. Because it is not a black body emitter, it does not have a CCT. However, it can be equated to a D illuminant with a CCT of 5455 K.
F-Series (Fluorescents)	F2	One of the six standard fluorescents. It is one of two semi-broadband fluorescent illuminants. F2 describes a cool white. Its CCT is 4230 K.
	F4	Used to calibrate the CIE color rendering index (CRI), F4 is a warm white simulator with CCT of 2940 K.
	F7	A broadband fluorescent illuminant, F7 is a D65 daylight simulator with a CCT of 6500 K.
	F11	A narrow tri-band fluorescent F11 has a CCT of 4000 K. F11 is sometimes known as TL84 for a non-CIE standard lamp made by Philips for the European market.
LEDs	LED-B1	CCT 2733 K, phosphor-converted blue
	LED-B2	CCT 2998 K, phosphor-converted blue
	LED-B3	CCT 4103 K, phosphor-converted blue
	LED-B4	CCT 5109 K, phosphor-converted blue
	LED-B5	CCT 6598 K, phosphor-converted blue
	LED-BH1	CCT 2851 K, mix of phosphor-converted blue LED and red LED, called a blue-hybrid
	LED-RGB1	CCT 2840 K, mix of red, green, and blue LEDs
	LED-V1	CCT 2724 K, phosphor-converted violet
	LED-V2	CCT 4070 K, phosphor-converted violet

[1] Kokka, A., et al. “Development of white LED illuminants for colorimetry and recommendation of white LED reference spectrum for photometry.” Metrologia, Vol. 55(4), p. 526. June 28, 2018

-------------------------------------------------------------------------------------------------------------------

Luminus Website https://www.luminus.com/

Luminus Product Information (datasheets): https://www.luminus.com/products

Luminus Design Support (ray files, calculators, ecosystem items: [power supplies, lenses, heatsinks]): https://www.luminus.com/resources

Luminus Product Information sorted by Applications:  https://www.luminus.com/applications

Where to buy Samples of Luminus LEDs: https://www.luminus.com/contact/wheretobuy.

--------------------------------------------------------------------------------------------------------------------

Technical Support Contact Information: [e-mail]

Sales Support Contact Information: [e-mail]

Customer Service Support Contact Information: [e-mail]